Analytical devices suitable for home testing of analytes are now widely commercially available. One lateral flow immunoassay device suitable for this purpose for the measurement of the pregnancy hormone human chorionic gonadotropin (hCG) is sold by Unipath under the brand-name CLEARBLUE® and is disclosed in EP291194.
In particular EP291194 discloses an immunoassay device comprising a porous carrier containing a particulate labelled specific binding reagent for an analyte, which reagent is freely mobile when in the moist state; and an unlabelled specific binding reagent for the same analyte, which reagent is immobilised in a detection zone or test zone downstream from the unlabelled specific binding reagent. Liquid sample suspected of containing analyte is applied to the porous carrier whereupon it interacts with the particulate labelled binding to form an analyte-binding partner complex. The particulate label is coloured and is typically gold or a dyed polymer, for example latex or polyurethane. The complex thereafter migrates into a detection zone whereupon it forms a further complex with the immobilised unlabelled specific binding reagent enabling the extent of analyte present to be detected or observed. Due to the nature of the binding reactions taking place it is necessary to wait for a particular period of time to elapse after the test has commenced in order to read the result. This is particularly important for a visual semi-quantitative type of test whereby the result develops over time as the fluid sample, which is typically in excess, continually flows past the detection zone resulting in the build-up of captured analyte complex
Such devices are simple to use, and the result may be determined visually, without the need for an electronic reader. They are also semi-quantitative, giving rise to either a positive or negative result. Due to the nature of the test, it is necessary to wait for a certain period of time such that a sufficient amount of the labelled analyte complex is captured in the detection zone. If a result were read too early, it might be interpreted as being negative, even though analyte were in fact present in the sample. In view of this, instructions requiring the user to wait for a prescribed period of time after the sample has been applied in order to view the result often accompany the test. Other methods include a signal that is generated after a particular period of time has elapsed which indicates to the user that a sufficient period of time has elapsed in order to read the test, as disclosed in our copending application no PCT/EP03/00274.
EP653625 discloses a lateral flow assay test-strip for use in combination with an assay reader the whereby the extent of binding of particulate label is determined optically (i.e. by assay reader). The reader further incorporates a timing mechanism such that the result is displayed after a preset period, thus removing the need for the user to time the test.
U.S. Pat. No. 5,837,546 discloses an integrated reader and test-strip wherein the test-strip is provided with additional electrodes which sense the presence of fluid on the test-strip which generates a signal to switch on the sensing electronics.
The above tests however, require a preset time to have elapsed before the result is read or displayed. This is not always convenient, for example in the case wherein the test in used in an emergency room procedure and the time to result is vital. For instance, some commercial lateral flow assay-based test kits for markers of cardiac damage require as much as 15 minutes or so for the assay to be completed. In other cases, such as a pregnancy test, there is a natural desire on the part of the user to have the result as soon as possible.